In recent years, increased integration of integrated circuits has posed the need to use microlithography processes capable of resolving smaller feature sizes. In view of the perceived resolution limits of optical microlithography, microlithographic apparatus and methods employing a charged-particle beam (e.g., electron beam or ion beam) have been the subject of intense development efforts. Considerable effort has also been expended to develop a practical X-ray microlithography system.
In electron-beam microlithography, it has been possible to form and use an electron beam tightly focused to several nanometers in diameter for "drawing" minute patterns having feature sizes of 1 .mu.m or less. During such drawing, the beam is scanned so as to trace the entire exposure pattern. As a result, a substantial amount of time is required to complete an exposure of an entire die on a sensitive substrate (e.g., a wafer) Thus, using an electron beam to "draw" features on a sensitive substrate has a much lower throughput than, e.g., projection microlithography. Also, if the pattern to be drawn comprises extremely small features, the diameter of the charged particle beam must be correspondingly smaller, which poses certain technical challenges.
Projection microlithography, including such microlithography employing a charged-particle beam, utilizes a mask or reticle ("mask") that defines the pattern to be transferred to a sensitive substrate. Because of certain limitations in simultaneously projecting the entire mask pattern using a charged particle beam, the mask field is typically divided into multiple "subfields" each defining a portion of the overall pattern. The subfields are normally all the same size and are separated from each other on the mask by struts that serve principally to provide mechanical rigidity to the mask. The subfields are scanned subfield-by-subfield and projected individually through a charged-particle-beam optical system onto corresponding "transfer subfields" on the substrate. The transfer subfields are projected onto the substrate in such a manner that regions of the mask occupied by the struts are not included in the exposure on the substrate; i.e., the transfer subfields are "stitched" together without struts to form a complete die.
For use with a charged-particle beam, one group of masks is termed "stencil masks", in which pattern regions through which the beam is intended to readily pass are defined as voids. A representative method known in the art for manufacturing a stencil mask is depicted stepwise in FIGS. 4(a)-4(f).
In a first step (FIG. 4(a)), a surface of a silicon substrate is doped with boron using, typically, thermal diffusion. Such doping forms a surficially doped silicon substrate 12 comprising boron-doped layer 13 and a silicon substratum 14.
In a second step (FIG. 4(b)), an electron beam is used to form a desired resist pattern on the surface of the boron-doped layer 13, which is subsequently etched to form corresponding voids 15 extending through the thickness of the boron-doped layer 13 to the substratum 14. The pattern-defining voids 15 are located in regions that are destined to be mask subfields.
In a third step (FIG. 4(c)), a silicon nitride layer 16 is applied to all surfaces, typically using a low-pressure chemical vapor deposition (LPCVD) technique. A pattern of openings 17 is then formed on the lower silicon nitride layer, wherein each such opening 17 corresponds to a respective future location of a mask subfield and is thus situated opposite a respective group of voids 15 destined to be within a respective mask subfield (FIG. 4(d)).
With the silicon nitride layer serving as a mask 18, the structure shown in FIG. 4(d) is immersed in an aqueous KOH solution that "wet etches" the silicon substratum 14 at each exposed location 17 (FIG. 4 (e)). Wet etching is allowed to progress depthwise through the substratum 14 to the boron-doped layer 13 to form voids 19 extending depthwise entirely through the thickness of the silicon substratum 14.
After the wet etching step, the structure shown in FIG. 4(e) is removed from the KOH solution and washed using a solution comprising nitric acid and hydrogen peroxide, rinsed with pure water, and allowed to dry. The silicon nitride layer 16 is then removed by a dry etching technique to complete formation of the stencil mask 20 (FIG. 4(f)). The stencil mask comprises struts 21 separating the subfields 22 from each other.
Unfortunately, when the nearly finished mask is removed from the aqueous KOH solution after completion of the wet etching step, surface tension commonly causes fracture of the mask, resulting in low manufacturing efficiency.